1. Field of the Invention
The present invention relates to apparatus which convert a DC supply voltage into an alternating voltage, and more particularly to power conversion equipment, such as power inverters used in motor drives that control electric energy supplied to an electric motor.
2. Description of the Related Art
Motor drives are commonly employed to control the application of electric energy to a three-phase induction motor. The motor drive includes a voltage source inverter which switches DC voltage to output lines in a pulse width modulated (PWM) manner so as to control the frequency and amount of voltage applied to the motor thereby controlling the motor speed.
In one type of an inverter, known as a two-level inverter, the output voltage waveform is formed by applying sequences of positive and negative voltage pulses to each terminal of the motor. For example, a three-phase, two-level inverter is made up of three poles. Each inverter pole comprises two semiconductor switches which selectively connect the respective phase output terminal to either a positive voltage bus or a negative voltage bus in order to produce the two voltage levels at that phase output terminal.
Any short term direct connection of the positive and negative voltage buses results in extremely high rate of current change, which if not detected in a very short time will cause severe overcurrent stress and eventual failure of a number of components in the direct path of the failure. Failure of a switch can result in damage to the switch, the pole, and other inverter components or to the motor. Therefore, it is desirable to detect such failures very quickly to be able to take prompt responsive action. Although current sensors have been located in the inverter output lines to the motor in order to regulate the current applied to the motor and to sense faults associated with output malfunctions, current sensors traditionally have not been placed elsewhere in the inverter branch circuits to detect malfunctions. Six or more additional current sensors would be needed in a three-phase inverter in order to detect failure of any one of the switching components.
Therefore, it is desirable to provide an improved apparatus and method for detecting malfunctions of a power inverter and providing diagnostic information that is useful in correcting the condition which lead to a malfunction.